Colorado’s Blizzard of 2003, One Year Later—
The Prediction Power of Fine-Scale Models

BOULDER —Weather forecasters may soon do a better job handling snowstorms
like the one that paralyzed Colorado’s Front Range last March, according
to an analysis by the University Corporation for Atmospheric Research
(UCAR), the National Oceanic and Atmospheric Administration (NOAA), and
colleagues in the private sector.

According to the study team, the fine detail from a new generation of
computer forecast models may help forecasters convey important town-to-town
differences in snowfall timing and intensity, like those observed during
the historic storm on March 16–20, 2003. These new tools—such
as the Weather Research and Forecasting Model, to be implemented later
this year by the National Weather Service (NWS)—will help meteorologists
include more detail in public forecasts.

Overall, forecasters did a good job alerting the public to last year’s
storm, according to the study's leader, Douglas Wesley (UCAR Cooperative
Program for Operational Meteorology, Education and Training). Computer
models indicated the storm’s general approach up to a week in advance.
Then, within two days of its arrival, they showed the potential for over
four feet of snow along Colorado’s Front Range. “The accuracy
of these model projections, and the public forecasts issued by the NWS,
were perhaps unprecedented,” says Wesley.

But neither human nor virtual forecasters predicted how dramatically the
storm would vary across the Front Range. “We found differences of
several feet of snowfall at similar elevations in a span of 15 miles or
less,” says Wesley. Just west of Longmont and Loveland, for instance,
only three to six inches of wet snow accumulated. Meanwhile, at the same
elevation on the south and west sides of Boulder and Denver, three to
four feet of snow piled up.

The huge disparities occurred in part because of subtle temperature differences.
As the massive storm spun in a counterclockwise direction, says Wesley,
“relatively warm air descended from the canyons of Boulder and Larimer
County, while cold northerly surface winds were blocked by the terrain.”
This helped keep readings at or just above freezing downstream from the
canyons, limiting the snowfall accumulations there.

In large-scale computer models currently used by the NWS, the atmosphere
is depicted at points separated horizontally by more than 10 miles. This
is much too great a distance to capture the small-scale variations in
temperature and precipitation that shaped the March 2003 snowstorm. To
get a better view, the UCAR-NOAA team put two finer-scale research models
to the task, each with less than 2 miles between grid points. The team
fed actual data from the storm into these two models, then watched how
well the models depicted the storm’s evolution.

One of the key elements in heavy Front Range snowfall is upslope flow,
where moist winds blow from east to west. Along with their sharper depiction
of the mountains themselves, the finer-scale computer models did a better
job depicting the upslope flow that slammed into the Front Range, says
Wesley. About a mile above Denver, the standard computer model showed
east-to-west flow of 15–20 miles per hour during the height of the
storm. The two finer-scale models showed speeds of 20–25 mph, closer
to the observed speed of 30–35 mph.

“The models’ abilities to capture the depth and strength of
the upslope flow are likely critical to their forecasts of low-level temperatures
and precipitation,” says Wesley. However, he notes, both models
failed to predict the full extent of the warmer air at ground level, where
light downslope winds kept some areas at or above freezing, even as howling
upslope winds fueled snowfall just above the surface.

“With the ongoing advances in computing speed that we are currently
seeing,” says Wesley, “forecast models with horizontal grid
spacing of less than five miles are becoming a realistic expectation in
a 5- to 10-year timetable. These enhanced models will become even more
important tools for forecasters in complex terrain anywhere in the world.”

This visible satellite
image of Colorado was taken on March 22, 2003, a couple of days
after the Front Range snowstorm ended. Several pockets of snow-free
ground are visible just to the east of the foothills north of
Denver, where downslope canyon winds kept some areas near or above
freezing. (Image courtesy COMET and NOAA.)

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